Heating water above 100° C. at 1 atmosphere will transform it into steam. In steam generating devices, such as steam irons, water is applied to a hot surface in order to generate the steam. However, the steam can form an insulating layer between the surface and the water droplets, thereby effectively slowing down the evaporation of water. The water droplets will tend to bounce on the surface instead of evaporating into steam. This effect is called the Leidenfrost effect and generally occurs above 160° C. This effect is for instance observed in steam irons.
Various methods have been proposed to prevent the Leidenfrost effect, ranging from providing special structures in the steam chamber, like ribs for instance, to the use of coatings on the surface of the steam chamber. A suitable steam promoter coating is hydrophilic and moderately heat-insulating. The moderately heat-insulating character of the coating slightly lowers the surface temperature in the absence of water and prevents the water from touching the hot aluminum substrate. When some water touches the surface, the surface is immediately cooled down effectively to below Leidenfrost effect temperatures. Preferably also, such steam promoter coatings do have a certain amount of porosity. By virtue of the hydrophilic character of the steam promoter coating, the water introduced spreads readily over the surface of the steam chamber. A suitable steam promoter coating offers a combination of good wetting, absorption of water into the porous structure, and a high surface roughness.
A steam generating device of the type described in the preamble is known from U.S. Pat. No. 3,499,237. The known device (a steam iron) is provided with a steam promoter coating composition, mainly composed of an alkali metal silicate compound and powdered glass. In particular sodium silicate (water glass) is used. Water glass can be dried to form a hard glassy layer. Due to its inorganic nature it is temperature resistant and can be used as a steam promoter coating in a steam iron. Due to its high pH, water glass etches the aluminum soleplate substrate, thereby improving the adhesion of the coating layer to the aluminum. A major drawback of water glass is its solubility in water, the reason being the high amount of alkali present in water glass. As soon as water is added to the steam chamber of a steam iron, the known steam promoter material will at least partly dissolve, and may leach out of the steam chamber. This effect is even more pronounced when the steam chamber is decalcified by rinsing it with water.